A symbol representing cardinality or number signs in set theory and numerical contexts.
Examples
Cardinality or size of a set
\#{A} = 5 \text{ means set A has 5 elements}
Number of elements notation
\text{The number of solutions is } \#{x \in \mathbb{R} : x^2 = 4} = 2
Counting elements in an index set
\#{i \in I : a_i \neq 0} < \infty